I've got a bass with an active pre-amp which has a nasty hum problem.
The pre-amp module is a "Ming Shi #9020 2BAND EQ (T.B) Made in China"
I found the wiring diagram for it and re-wired the whole thing.
I built a faraday cage out of thin brass sheet to completely surround the pot-bath. I even added shielding braid to cover the wires right up to the pickups.
Still,..... no joy.
Now, it could just be particularly noisy, nasty, Chinese pickups being amplified. But I don't know how you could make a pickup so poorly as to create this much hum.
I've just noticed that if I touch the potting mix in the back of the pre-amp module, (and nothing else) the noise goes way up, even more. This doesn't make sense to me, unless the potting mixture has become conductive over time. (like the problem with the voice modules in a Roland MKS-30, and other synths)
Does anyone else think this is likely, or have I missed something?
Any help would be greatly appreciated._________________What makes a space ours, is what we put there, and what we do there.

I built a faraday cage out of thin brass sheet to completely surround the pot-bath. I even added shielding braid to cover the wires right up to the pickups.
Still,..... no joy.

Hi uncle
i tried that once too wasnt great.
Stewart Mac has some self adhesive conductive tape for shielding which is really good. If you are shielding the cavity dont run a string ground

Quote:

Now, it could just be particularly noisy, nasty, Chinese pickups being amplified. But I don't know how you could make a pickup so poorly as to create this much hum.

Haha just get me to wind one for you . but probably not the issue

Quote:

I've just noticed that if I touch the potting mix in the back of the pre-amp module, (and nothing else) the noise goes way up, even more. This doesn't make sense to me, unless the potting mixture has become conductive over time. (like the problem with the voice modules in a Roland MKS-30, and other synths)

yeah that could be it , at least i wouldnt be surprised if it was

one thing that occured to me is on active systems the control pots used are generally around 25K not the more usual 250K/500K but it depends on the system used, was the preamp original or an add on ? have the pots ever been changed ?_________________In an infinite universe one might very well
ask where the hell am I
oh yeah thats right the land of OZ
as good an answer as any

Its unlikely to fix the noise issue
Its more of a safety thing ,I prefer to shield cavities properly than have a string ground . with a string ground its very likely in the event of an earthing fault the player may actually become part of the earthing system
say grabbing a mike whilst holding the neck of the instrument,
if there is any potential differance between the two the player becomes
a conductor and sometimes as has happened a very dead conductor.
just better to avoid string grounds whenever possible.

I would be more inclined towards your suspicion of the active circuitry
potting going bad. hmm is there any exposed metal on the active circuitry casing ? if so maybe try earthing that back to the jack ground.
A picture or 2 to might be good._________________In an infinite universe one might very well
ask where the hell am I
oh yeah thats right the land of OZ
as good an answer as any

Have you completely ruled out the jack? A lot of inexpensive basses with active pickups use Switchcraft style clone jacks....usually used for end pin jacks in acoustics, and the internal contacts tend to break very easily. Typically the jack will be a stereo jack with the 'ring' contact providing the switch to turn the electronics on. This is done by connecting the negative lead (or common) to ground when a guitar plug is inserted into the jack. Make sure the jack is wired properly too....I've seen them wired incorrectly as well. Try bypassing it completely if you're not sure and want to rule it out....

Kind of a long shot here ,but try turning your soldering iron off.
I was building a spring reverb,and testing all it's sub circuits in the chassis ,on top of my work bench .i spent hours reworking the ground scheme ; trying to get rid of a horrendous hum. Finally figured out that my adjustable solder station emits a prodigious EMF field. Turned it off & it was all quiet ._________________"It's nice 2 B important; but it's important 2 B nice"

Well ... Just for yukes , try puting an insulator over the potting ( like a chunk of plastic from a milk jug) , then wrap the thing in a sheild ( like foil or metal tape)
Try it grounded & un grounded .
That might help._________________"It's nice 2 B important; but it's important 2 B nice"

I've got a bass with an active pre-amp which has a nasty hum problem.
The pre-amp module is a "Ming Shi #9020 2BAND EQ (T.B) Made in China"
I found the wiring diagram for it and re-wired the whole thing.
I built a faraday cage out of thin brass sheet to completely surround the pot-bath. I even added shielding braid to cover the wires right up to the pickups.
Still,..... no joy.
Now, it could just be particularly noisy, nasty, Chinese pickups being amplified. But I don't know how you could make a pickup so poorly as to create this much hum.
I've just noticed that if I touch the potting mix in the back of the pre-amp module, (and nothing else) the noise goes way up, even more. This doesn't make sense to me, unless the potting mixture has become conductive over time. (like the problem with the voice modules in a Roland MKS-30, and other synths)
Does anyone else think this is likely, or have I missed something?
Any help would be greatly appreciated.

man, PLEASE, can you send me those wiring diagrams? I REALLY need this.

if you can, please send to my e-mail, its: cidinhodc(at)hotmail(dot)com

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